1. Field of the Invention
The present invention relates generally to a transmission control system for motor vehicles, and more particularly to a fluid pressure control system for motor vehicle transmissions.
2. Description of the Prior Art
Generally, in order to accomplish operation of the power train for motor vehicle transmissions, it is preferable that the hydraulic pressure for actuating the frictional engaging means be correspondingly adapted to the driving speed of the vehicle, that is, the hydraulic pressure be sufficiently high so as to transmit a substantial amount of torque when the vehicle is driven at low speed and be substantially low so as to transmit a small amount of torque when the vehicle is driven at a high rate of speed. Additionally, it is desirable that the hydraulic pressure applied to the frictional engaging means be gradually increased in order to decrease or avoid any shocks resulting from the transmission gear shifts when the driving ratio is changed from low speed to high speed or vice versa.
In order to satisfy the above-noted requirements, a conventional transmission control system has heretofore involved the use of two independent governor pressures, one of which, having a greater pressure or value, being applied to the regulator valve when the vehicle is driven within a low drive ratio mode, while the other governor pressure, having a smaller pressure or value, is applied to the regulator valve when the vehicle is driven within a high drive ratio mode. Such a system however, is nevertheless not totally satisfactory due to the fact that the presence of a plurality of governor pressures results in a substantial increase in the complexity of the control structure or the machining configurations required of the regulator valve and conduits through which the governor pressures are transmitted. More particularly, the regulator valve is required to utilize a spool piston having a substantial number of lands and must also be made with a high degree of accuracy.